Gapless microlens array and method of fabrication

Abstract

A microlens array with reduced or no empty space between individual microlenses and a method for forming the same. The microlens array is formed by patterning a first set of microlens precursors in a checkerboard pattern on a substrate. The first set of microlens precursors is reflowed and cured into first microlenses impervious to subsequent reflows. Then, a second set of microlens precursors is patterned in spaces among the first microlenses, reflowed and cured into second microlenses. The reflows and cures can be conducted under different conditions, and the microlenses may be differently sized. The conditions of the reflows can be chosen to ensure that the focal lengths of microlenses are optimized for maximum sensor signal.

1. An imaging device, comprising:
 an imager having a pixel cell array arranged in a plurality of rows and columns, the pixel cell array comprising:
 at least first, second, and third pixel cells having corresponding first, second, and third photosensors, the first and second photosensors having a first distance between each other, and the second and third photosensors having a second distance between each other, the first and second distances being different from one another, and 
 a substantially gapless microlens array formed over the pixel cell array, the microlens array having at least first, second, and third microlenses at least two of said microlenses having respective focal points corresponding to the first and second photosensors of the pixel cell array such that photo radiation striking a top surface of at least one microlens at an orthogonal angle does not strike the corresponding photosensor at an orthogonal angle after passing through the at least one microlens, and 
 
 a plurality of row and column lines coupled to the pixel cell array; and 
 readout circuitry coupled to the plurality of row and column lines. 
 
   
   
     2. The imaging device of  claim 1 , further comprising a color filter array between the pixel cell array and the microlens array. 
   
   
     3. The imaging device of  claim 2 , wherein the color filter array comprises at least first, second, and third color filters. 
   
   
     4. The imaging device of  claim 3 , wherein the first, second, and third color filters correspond to blue, green, and red color filters. 
   
   
     5. The imaging device of  claim 1 , wherein each microlens of the microlens array has a focal plane that is substantially the same. 
   
   
     6. The imaging device of  claim 1 , wherein at least one microlens of the microlens array has a focal plane that is different from other microlenses of the array. 
   
   
     7. The imaging device of  claim 1 , wherein the first distance is greater than the second distance. 
   
   
     8. The imaging device of  claim 1 , further comprising fourth and fifth pixel cells having fourth and fifth photosensors, and wherein a common storage node is shared by at least two of the first, second, and third photosensors and the fourth and fifth photosensor. 
   
   
     9. A method of forming a pixel cell array, comprising:
 forming an array of photosensors having at least one photosensor spaced unequally from surrounding photosensors; 
 forming a first set of microlens precursors having a checkerboard pattern over a substrate formed over the photosensors; 
 reflowing and curing the first set of microlens precursors; 
 forming a second set of microlens precursors over the substrate; 
 reflowing and curing the second set of microlens precursors such that the first and second microlens precursors form a substantially gapless microlens array having at least first, second, and third microlenses having a focal point corresponding to first and second photosensors in the photosensors array. 
 
   
   
     10. The method of  claim 9 , wherein the first and second set of microlens precursors are reflowed under respective first and second reflow conditions. 
   
   
     11. The method of  claim 10 , wherein the first and second reflow conditions are the same. 
   
   
     12. The method of  claim 10 , wherein the first reflow condition is selected to form first microlenses having a first focal length corresponding to a first wavelength of light and the second reflow condition is selected to form second microlenses having a second focal length corresponding to a second wavelength of light. 
   
   
     13. The method of  claim 12 , wherein the first and second wavelengths are different. 
   
   
     14. The method of  claim 9 , further comprising:
 forming a third set of microlens precursors over the substrate; and 
 reflowing and curing the third set of microlens precursors. 
 
   
   
     15. The method of  claim 9 , further comprising coupling at least two of the photosensors to a common node. 
   
   
     16. The method of  claim 9 , wherein the first set of microlens precursors are formed by the steps comprising:
 depositing a material layer over the substrate; 
 exposing selected areas of the material layer to developing conditions; and 
 removing the non-selected areas of the material layer. 
 
   
   
     17. An imager, comprising:
 a pixel cell array arranged in a plurality of rows and columns, the pixel cell array comprising: 
 at least first, second, and third pixel cells having corresponding first, second, and third photosensors, the first and second photosensors having a first distance between each other, and the second and third photosensors having a second distance between each other, the first and second distances being different from one another, and 
 a substantially gapless microlens array formed over the pixel cell array, the microlens array having at least first, second, and third microlenses having first, second, and third focal points at different depths of corresponding first, second, and third, photosensors, each depth corresponding to first, second, and third colors, and 
 a plurality of row and column lines coupled to the pixel cell array; and 
 readout circuitry coupled to the plurality of row and column lines. 
 
   
   
     18. The imager of  claim 17 , further comprising a color filter array between the pixel cell array and the microlens array. 
   
   
     19. The imager of  claim 17 , wherein the color filter array comprises at least first, second, and third color filters. 
   
   
     20. The imager of  claim 17 , wherein the first, second, and third color filters correspond to blue, green, and red color filters. 
   
   
     21. The imager of  claim 17 , wherein the first distance is greater than the second distance. 
   
   
     22. A pixel cell array, comprising:
 at least first, second, and third pixel cells having corresponding first, second, and third photosensors, the first and second photosensors having a first distance between each other, and the second and third photosensors having a second distance between each other, the first and second distances being different from one another, and 
 a substantially gapless microlens array formed over the at least first, second, and third pixel cells, the microlens array having at least first, second, and third microlenses at least two of said microlenses having respective focal points corresponding to the first and second photosensors of the pixel cell array such that photo radiation striking a top surface of at least one microlens at an orthogonal angle does not strike the corresponding photosensor at an orthogonal angle after passing through the at least one microlens. 
 
   
   
     23. The pixel cell array of  claim 22 , further comprising a color filter array between the at least first, second, and third pixel cells and the microlens array. 
   
   
     24. The pixel cell array of  claim 23 , wherein the color filter array comprises at least first, second, and third color filters. 
   
   
     25. The pixel cell array of  claim 24 , wherein the first, second, and third color filters correspond to blue, green, and red color filters. 
   
   
     26. The pixel cell array of  claim 22 , wherein each microlens of the microlens array has a focal plane that is substantially the same. 
   
   
     27. The pixel cell array of  claim 22 , wherein at least one microlens of the microlens array has a focal plane that is different from other microlenses of the array. 
   
   
     28. The pixel cell array of  claim 22 , further comprising fourth and fifth pixel cells having fourth and fifth photosensors, and wherein a common storage node is shared by at least two of the first, second, and third photosensors and the fourth and fifth photosensor.